They had a few comic cross overs (JLA/Avengers, X-men/Teen Titans, Marvel vs.DC) all were horrible cash grab stories, so making movie or show would not be a good ideas.
I love Batman and but I'm more of a Marvel fan. I love the Avengers, Xmen and Spidey. Deadpools awesome too.
I agree with PP on the cross overs being cash grab stories. I couldn't see a film working out successfuly. I can enjoy both DC and Marvel without them coming together.
I'm personally more of a DC fan. Marvel ok tho. I hope that Disney buys DC too. That way the worlds and continuities can be combined. Who would live a nightwing and daredevil team up?